The present invention relates to signal compensation in color video signal processing. Particularly, the present invention relates to a color signal compensation apparatus applicable to reproduction of colors in the details of an image and enhancement of the outline of the image displayed on a screen of a television.
A conventional color signal compensation apparatus is disclosed in, such as, U.S. Pat. No. 4,504,853 and shown in FIG. 1.
In this conventional apparatus, the outline of the image which is the edge where color is changed, is compensated by means of correlation of a color subcarrier signal (hereinafter called as a C-signal) and a base band luminance signal (hereinafter called as a Y-signal).
This compensation is applied to the C-signal whose frequency band is restricted so that transition duration of color edge is degraded, for example due to reproduction by a home-use video tape recorder.
However, there is a drawback in the conventional apparatus. Let us see when the apparatus is applied to reproduction of the image in which both Y-signal and C-signal are changed in step like, in pulse like and in narrow notched pulse like in the same timing. See FIG. 2(1), (2) and (3).
Waveforms of original Y-signal and C-signal before reproduction are shown in FIG. 2(a) and (b), respectively. When these signals are passed through a transmission system such as a video tape recorder in which a frequency band for the C-signal is restricted, their waveforms become as shown in FIG. 2(c) and (d), respectively.
These transmitted Y- and C-signals are applied to a low pass filter 112 and a band pass filter 104 of the apparatus shown in FIG. 1.
Processing of the Y-signal is explained first. The output of the low pass filter 112 is shown in FIG. 2(e). This waveform becomes as shown in FIG. 2(f) when processed by a first differentiator 114 and a full-wave rectifier 116. The waveform shown in FIG. 2(f) becomes as shown in FIG. 2(g) when processed by a second differentiator 118. The waveform shown in FIG. 2(g) then becomes as shown in FIG. 2(h) when processed by a limiter 120.
Next, the output of a first adder 110 is shown in FIG. 2(i) after the C-signal applied to the band pass filter 104 has been processed by a delay matching network 106, a one-half subcarrier period delay circuit 108 and the first adder 110. The signal of waveform shown in FIG. 2(i) is then applied to a four-quadrant multiplier 124. Also applied to the multiplier 124 is the signal produced by processing the output of the limiter 120 shown in FIG. 2(h) by a dual ramp generator 122, which is multiplied by the output of the first adder 110 shown in FIG. 2(i). Then, a second adder 126 produces a compensated output signal shown in FIG. 2(k).
Among compensated signals shown in FIG. 2(k), first in the case of FIG. 2(l), the C-signal whose frequency band has been restricted as shown in FIG. 2(d) is reproduced as its waveform is similar to that of the original C-signal. This is apparent when FIG. 2(b) and (k) are compared to each other.
On the other hand, in the case of FIG. 2(2) and (3), the compensated waveforms shown in FIG. 2(k) are not similar to those of the original C-signals shown in FIG. 2(b), respectively.
As has been disclosed above, according to the conventional apparatus, it is possible to reproduce frequency components in a high frequency band degraded due to restriction of a frequency band in the case where an original signal comprises frequency components in a low to high frequency band, corresponding to such as the outline of an image.
However, a pulse-like image in which frequency components in a high frequency band are only included can not be well reproduced. Because, repeated signal processing of a narrow frequency band such as in recording and reproduction of video signals results in degradation of details of a C-signal so that an edge where color is changed is hardly detected. Colors changed by turns like a stripe and change of light and shade of a single color thus can not be well reproduced. Therefore, the conventional apparatus is insufficient for frequency band improvement of a C-signal in above case.